Iron is an essential trace element for almost all organisms and is relevant in particular with respect to growth and the formation of blood. The balance of the iron metabolism is in this case primarily regulated on the level of iron recovery from haemoglobin of ageing erythrocytes and the duodenal absorption of dietary iron. The released iron is taken up via the intestine, in particular via specific transport systems (DMT-1, ferroportin), transferred into the blood circulation and thereby conveyed to the appropriate tissues and organs (transferrin, transferrin receptors).
In the human body, the element iron is of great importance, inter alia for oxygen transport, oxygen uptake, cell functions such as mitochondrial electron transport, cognitive functions, etc. and ultimately for the entire energy metabolism.
On average, the human body contains 4 to 5 g iron, with it being present in enzymes, in haemoglobin and myoglobin, as well as depot or reserve iron in the form of ferritin and hemosiderin. Approximately half of this iron, about 2 g, is present as heme iron, bound in the haemoglobin of the erythrocytes. Since these erythrocytes have only a limited lifespan (75-150 days), new ones have to be formed continuously and old ones degraded (over 2 million erythrocytes are being formed per second). This high regeneration capacity is achieved by macrophages phagocytizing the ageing erythrocytes, lysing them and thus recycling the iron thus obtained for the iron metabolism. The majority of the iron required for erythropoiesis, about 25 mg per day, is provided in this way.
The daily iron requirement of a human adult is between 0.5 to 1.5 mg per day, infants and women during pregnancy require 2 to 5 mg of iron per day. The daily iron loss, e.g. by desquamation of skin and epithelial cells, is low. Increased iron loss occurs, for example, during menstrual hemorrhage in women. Generally, blood loss can significantly reduce the iron level since about 1 mg iron is lost per 2 ml blood. In a healthy human adult, the normal daily loss of iron of about 1 mg is usually replaced via the daily food intake thus rebalancing the daily iron requirement to the adequate level
The iron level is regulated by absorption, with the absorption rate of the iron present in food being between 6 and 12%, and up to 25% in the case of iron deficiency. The absorption rate is regulated by the organism depending on the iron requirement and the size of the iron store. In the process, the human organism utilizes both divalent as well as trivalent iron ions. Usually, iron(III) compounds are dissolved in the stomach at a sufficiently acid pH value and thus made available for absorption. The absorption of the iron is carried out in the upper small intestine by mucosal cells. In the process, trivalent non-heme iron is first reduced in the intestinal cell membrane to Fe(II) for absorption, for example by ferric reductase (membrane-bound duodenal cytochrome b), so that it can then be transported into the intestinal cells by means of the transport protein DMT1 (divalent metal transporter 1). In contrast, heme iron enters the enterocytes through the cell membrane without any change. In the enterocytes, iron is either stored in ferritin as depot iron, or released into the blood by the transport protein ferroportin. Hepcidin plays a central role in this process because it is the essential regulating factor of iron absorption. The divalent iron transported into the blood by ferroportin is converted into trivalent iron by oxidases (ceruloplasmin, hephaestin), the trivalent iron then being transported to the relevant places in the organism by transferrin (see for example “Balancing acts: molecular control of mammalian iron metabolism”. M. W. Hentze, Cell 117, 2004, 285-297).
Mammalian organisms are unable to actively discharge iron. The iron metabolism is substantially controlled by hepcidin via the cellular release of iron from macrophages, hepatocytes and enterocytes.
Hepcidin is a peptide hormone produced in the liver. The predominant active form has 25 amino acids (see for example: “Hepcidin, a key regulator of iron metabolism and mediator of anaemia of inflammation”. T. Ganz, Blood, 102, 2003, 783-8), although two forms which are shortened at the amino end, hepcidin-22 and hepcidin-20, have been found. Hepcidin acts on the absorption of iron via the intestine and via the placenta and on the release of iron from the reticuloendothelial system. In the body, hepcidin is synthesized in the liver from what is known as pro-hepcidin, pro-hepcidin being coded by the gene known as the HAMP gene. The formation of hepcidin is regulated in direct correlation to the organisms iron level, i.e. if the organism is supplied with sufficient iron and oxygen, more hepcidin is formed, if iron and oxygen levels are low, or in case of increased erythropoiesis less hepcidin is formed. In the small intestinal mucosal cells and in the macrophages hepcidin binds with the transport protein ferroportin, which conventionally transports the phagocytotically recycled iron from the interior of the cell into the blood.
The transport protein ferroportin is a transmembrane protein consisting of 571 amino acids which is formed in the liver, spleen, kidneys, heart, intestine and placenta. In particular, ferroportin is localized in the basolateral membrane of intestinal epithelial cells. Ferroportin bound in this way thus acts to export the iron into the blood. In this case, it is most probable that ferroportin transports iron as Fe2+. If hepcidin binds to ferroportin, ferroportin is transported into the interior of the cell, where its breakdown takes place so that the release of the phagocytotically recycled iron from the cells is then almost completely blocked. If the ferroportin is inactivated, for example by hepcidin, so that it is unable to export the iron which is stored in the mucosal cells, the stored iron is lost with the natural shedding of cells via the stools. The absorption of iron in the intestine is therefore reduced, when ferroportin is inactivated or inhibited, for example by hepcidin. In addition, ferroportin is markedly localized in the reticuloendothelial system (RES), to which the macrophages also belong. Hepcidin plays an important part here when iron metabolism is impaired by chronic inflammation. In case of inflammation in particular interleukin-6 is increased, triggering an increase in hepcidin levels. As a result, more hepcidin is bound to the ferroportin of the macrophages, thus blocking the release of stored iron, which ultimately leads to anemia of inflammation (ACD or AI).
On the other hand, if the serum iron level decreases, hepcidin production in the hepatocytes of the liver is reduced so that less hepcidin is released and accordingly less ferroportin is inactivated, allowing a larger amount of stored iron to be transported into the serum.
Therefrom it becomes apparent that the hepcidin-ferroportin system directly regulates the iron metabolism and that a disorder of the hepcidin regulation mechanism therefore has a direct effect on iron metabolism in the organism. In principle the hepcidin-ferroportin regulation mechanism acts via the two following opposite principles:
On the one hand, an increase of hepcidin leads to inactivation of ferroportin, thus blocking the release of stored iron from the cells into the serum, thus decreasing the serum iron level. In pathological cases a decreased serum iron level leads to a reduced hemoglobin level, reduced erythrocyte production and thus to iron deficiency anemia.
On the other hand, a decrease of hepcidin results in an increase of active ferroportin, thus allowing an enhanced release of stored iron and an enhanced iron uptake e.g. from the food, thus increasing the serum iron level. In pathological cases an increased iron level leads to iron overload.
Iron overload states and diseases are characterized by excess iron levels. Therein, the problems arise from excess serum iron level which lead to non-transferrin bound iron (NTBI). The NTBI is rapidly taken up unspecifically by the organs, leading to an accumulation of iron in tissue and organs. Iron overload causes many diseases and undesired medical conditions, including cardiac, liver and endocrine damage. Further, iron accumulation in brain has been observed in patients suffering from neurodegenerative diseases such as for example Alzheimer's disease and Parkinson's disease. As a particular detrimental aspect of excess free iron the undesired formation of radicals must be mentioned. In particular iron(II) ions catalyze the formation (inter alia via Fenton reaction) of reactive oxygen species (ROS). These ROS cause damage to DNA, lipids, proteins and carbohydrates which has far-reaching effects in cells, tissue and organs. The formation of ROS is well known and described in the literature to cause the so-called oxidative stress.
A well-established hitherto existing method for treating iron overload is based on the concept to reduce the amount of iron in the serum by increased removal of the iron from the body. The eldest known and still routine treatment method in an otherwise-healthy person consists of regularly scheduled phlebotomies (bloodletting). When first diagnosed, the phlebotomies are usually scheduled fairly frequent, e.g. once a week, until iron levels are brought to within normal range, followed by phlebotomies which are then scheduled once a month or every three months depending upon the patient's rate of iron loading.
For patients unable to tolerate routine blood draws, there are chelating agents available for use. For example, deferoxamine (also known as desferrioxamine B, N′-{5-[acetyl(hydroxy)amino]pentyl}-N-[5-({4-[(5-aminopentyl)(hydroxy)amino]-4-oxobutanoyl} amino)pentyl]-N-hydroxysuccinamide or Desferal®), which is a bacterial siderophore, is an established drug used in chelation therapy. Deferoxamine binds iron in the bloodstream as an chelator and enhances its elimination via urine and faeces. Typical treatment of chronic iron overload requires subcutaneous injection over a period of 8-12 hours daily. Parenterally injectable compositions of desferrioxamine-B salts are described for example in WO 1998/25887.
Two newer drugs, licensed for use in patients receiving regular blood transfusions to treat thalassemia, resulting in the development of iron overload, are deferasirox and deferiprone.
Deferasirox (Exjade®, 4-(3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl)benzoic acid), being described for example in WO 1997/49395 and deferiprone (Ferriprox®, 3-hydroxy-1,2-dimethylpyridin-4(1H)-one) are similarly acting as an iron chelating agent, thus being suitable as a drug for iron chelation therapy.
Further compounds acting as iron chelator for use in the treatment of iron overload have been described. For example WO 2013/142258 relates to encapsulated particles of diethylenetriaminepentaacetate (DTPA) and a zinc salt. WO 2003/041709 relates to 4-hydroxy-2-alkylquniolines such as 4-hydroxy-2-nonylqunioline as an iron chelator. WO 1998/09626 relates to chelating agents for treating iron overload states on the basis of dithiocarbamate-containing compositions.
WO 2015/077655 relates to desferrithiocin derivatives of the formula (A) or (J)
for the use in the treatment of iron overload diseases. According to WO 2015/077655 said desferrithiocin derivatives have been found to act as iron chelating agents.
WO 2005/051411 relates to novel antibiotics or antimycotics on the basis of oxachelin and derivatives thereof according to formula
which are described to act as an iron chelator and to be used in the treatment of iron overload diseases.
The disadvantage in the treatment of iron overload by chelation therapy is the removal of the chelated iron from the body when the iron overload has already occurred instead of preventing the occurrence of the disorder. Further, the established drugs for iron chelation therapy are known to exhibit a toxic potential.
Modern approaches can be expected to supersede this method increasingly, in particular with increasing knowledge about the underlying mechanisms and development of appropriate treating methods on the basis of such knowledge. Hepcidin agonists or compounds which have an inhibiting or supporting effect on the biochemical regulatory pathways in the iron metabolism are basically known from the prior art.
Iron overload may occur, for example, if hepcidin expression is prevented, for example due to a genetic defect, such as in the known iron overload disease haemochromatosis. Hemochromatosis is a disease of iron overload caused by mutations in genes that control hepcidin synthesis or in the hepcidin gene itself. Low or absent levels of hepcidin in these patients result in enhanced amounts of active ferroportin, allowing increased absorption of dietary iron, leading to severe iron overload, which causes cardiac, liver and endocrine damages. Hepcidin mimetic peptides, i.e. peptides which similarly bind and inactivate ferroportin, have been shown to effectively reverse the accumulation of tissue iron in the hepcidin knockout mouse, a model of Type 2 (juvenile) hemochromatosis. (Ramos et al, Blood 2012).
In the known iron overload disease beta-thalassemia a mutation in the beta globin gene causes a reduction in hemoglobin production and ineffective erythropoiesis, the inability to produce adequate numbers of red cells because of damage to and death of developing red cells in the bone marrow. This causes upregulation of the rate of erythropoiesis and a reduction in hepcidin level to make more iron available for increased erythropoietic activity. This maladaptive response results in iron overload due to the reduced hepcidin levels, which lead to enhanced amounts of active ferroportin, allowing increased absorption of dietary iron, as described above. Red cells in thalassemia have a shortened half-life because of the toxicity of an imbalanced ratio of alpha- and beta-hemoglobin-subunits. Also in the treatment of beta-thalassemia the use of hepcidin mimetic peptides has been described, the therapeutic rationale being based on the increase of hepcidin activity leading to iron restriction and reduction of iron mediated damage in red cells. Administration of hepcidin mimetic peptides to the th3/+ mouse, a model of non-transfusion dependent beta-thalassemia resulted in relief of ineffective erythropoiesis, increased red cell survival time and improvement of anemia. In this model the prevention of iron overload due to reduction in the absorption of dietary iron turned out as an additional benefit of the hepcidin mimetic therapy (Gardenghi et al, 2010; Casu et al 2013).
The described therapeutic approaches are based on a direct involvement into the disturbed iron metabolism pathway by directly acting via the primary regulator hepcidin by providing a hepcidin mimetic or a hepcidin agonist, i.e. acting in the sense of a kind of hepcidin substitute or supply. The approach is based on the therapeutic rationale to treat iron overload, i.e. excess serum iron level, by inhibiting ferroportin, via the hepcidin-inactivation mechanism, thus blocking excessive iron absorption.
Further known iron overload related diseases are diseases associated with ineffective erythropoiesis such as the myelodysplastic syndromes (also known as MDS or myelodysplasia), polycythemia vera, etc.
Further, mutations in genes involved in sensing the systemic iron stores, such as hepcidin (Hamp1), hemochromatosis protein (HFE), hemojuvelin (HJV) and transferrin receptor 2 (TFR2) cause iron overload in mice and men. Accordingly, diseases related to HFE and gene mutations, chronic hemolysis associated diseases, sickle cell diseases, red cell membrane disorders, as well as Glucose-6-phosphate dehydrogenase deficiency (G6PD deficiency), erythropoietic porphyria and Friedrich's Ataxia can be mentioned. Further, subgroups of iron overload comprise transfusional iron overload, iron intoxication, pulmonary hemosiderosis, osteopenia, insulin resistance, African iron overload, Hallervordan Spatz disease, hyperferritinemia, ceruloplasmin deficiency, neonatal hemochromatosis and red blood cell disorders comprising thalassemia, alpha thalassemia, thalassemia intermedia, sickle cell disease and myelodysplastic syndrome are included.
Further disease and/or disorders and/or diseased conditions associated with elevated iron levels include, but are not limited to, diseases with elevated iron level, comprising ataxia, Friedrich's ataxia, age-related macular degeneration, age-related cataract, age-related retinal diseases and neurodegenrative disease, whereby such neurodegenrative disease comprises Alzheimer's disease, Parkinson's disease, pantothenate kinase-associated neurodegeneration, restless leg syndrom and Huntington's disease,
Hepcidin is a host defense peptide, representing a component of the innate immune system that responds to invading organisms. It has been described that many bacteria are highly dependent on a supply of iron from the host (so-called siderophilic organisms) and have evolved mechanisms to capture iron from the local tissues. The ability to limit the amount of iron available to such organisms by ferroportin-inhibitors may represent effective adjunctive therapy. One such siderophilic organism is Vibrio vulnificus, which causes rare but extremely severe infections in coastal communities, often in subjects with undiagnosed iron overload. Studies in animals that have been inoculated with a lethal dose of Vibrio vulnificus have demonstrated nearly 100% survival in response to treatment with hepcidin mimetic peptides, inactivating ferroportin, regardless of whether treatment is started before or after the infection is initiated (Arezes et al 2015).
As known hepcidin mimetics the so-called minihepcidins can be mentioned, described for example in WO 2013/086143. Minihepcidins are small-sized synthetic peptide analogues of the hepcidin N-terminus which is crucial for hepcidin interaction with ferroportin. Minihepcidins have been developed on the basis that the first 9 amino acids of hepcidin (DTHFPICIF) have been found to be sufficient for in vitro activity (measured as ferroportin-GFP degradation). Minihepcidins have a modified hepcidin-9 amino acid sequence to exhibit improved resistance to proteolysis and enhanced biophysical interaction with ferroportin. Minihepcidins are described to be useful for the treatment of human iron overload conditions caused by hepcidin deficiency.
WO 2015/069660 describes methods for increasing hepcidin expression for treating iron overload disorders by decreasing non-transferrin bound iron (NTBI) by administering a modified iron binding/releasing transferrin.
All the described compounds which act as hepcidin agonists, hepcidin mimetics or ferroportin inhibitor etc. are relatively high molecular weight compounds, in particular those which are obtainable predominantly by genetic engineering. Various further approaches on the basis of biomolecular interactions and biomolecules have been described. The disadvantage is the complex preparation and high sensitivity of such biomolecular compounds. In particular methods on the basis of ferroportin antibodies are not sufficiently efficient as the antibody-inhibited ferroportin is permanently reproduced by the organism and the inhibition is thus not sufficiently long-lasting to achieve the desired therapeutic effect.
Low molecular weight compounds which play a part in iron metabolism and can have an inhibiting or promoting effect are also known.
For example WO 2008/151288, WO 2008/118790, WO 2008/115999, and WO 2008/109840 describe compounds acting as divalent metal transporter-1 (DMT1) inhibitors and their use for the treatment of iron disorders such as thalassemia or hemochromatosis.
WO 2008/123093 relates to an agent for prevention or treatment of iron overload disorders, comprising 22 beta-methoxyolean-12-ene-3 beta,24(4 beta)-diol.
EP 1074254 and EP1072265 relate to the use of catechic- and flavonoid-structure plant polyphenols for treating iron overload.
WO 2011/029832 relates to thiazol and oxazol compounds, which act as hepcidin antagonists and are thus described to be suitable in the use for the treatment of iron deficiency diseases. Therein, hepcidin antagonistic activity is described to inhibit the inhibition of ferroportin by hepcidin, which is the opposite effect as has been found by the inventors of the present invention for the compounds as described herein.
Chemical compounds based on the structures of the general formulae of the present invention have hitherto not been disclosed in connection with their activity as ferroportin inhibitors or for the use in the prophylaxis and treatment of iron metabolism disorders which are associated with increased iron levels such as iron overload.
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